1. Field of the Invention
The present invention relates to a semiconductor device, and more particularly, to a refresh circuit which adjusts a refresh interval of a semiconductor memory device according to the position of a semiconductor memory device or a group of memory devices to which the semiconductor memory device belongs in a memory system having a plurality of semiconductor memory devices, and a method thereof.
2. Description of the Related Art
As a semiconductor memory device and a system including the semiconductor memory device develop, the number of memory devices included in a single module or memory system is gradually increased. For example, for a computer, one or more memory modules including a plurality of DRAM chips are included.
To embody a large capacity memory module, a variety of schemes are used, for example, the DRAM chips are mounted on the upper and lower sides of the module or a plurality of DRAM chips are sequentially stacked.
When several DRAM chips are included in a single module or several memory modules are included in a single system, heat of the DRAM chip can be a serious problem. In particular, when a plurality of DRAM chips are positioned in a narrow area, the temperature of a chip varies according to the position thereof.
For example, for a memory module having a stack structure, thermal dissipation of the DRAM chip located on the lower side is inferior to that of the DRAM chip located on the upper side. To solve this problem, a heat spreader or heat sink exhibiting low heat resistance can be installed on the surface of the DRAM chip located on the upper side of a memory module. However, it is still difficult to overcome a temperature difference according to the position of the DRAM chip. Thus, when a plurality of memory chips is located on one or more modules, the temperature difference exists according to the position of the memory chip.
A DRAM stores data in a cell capacitor. That is, data is stored in a capacitor of a DRAM cell in form of electric charges. However, since the capacitor is not perfect, the stored electric charges are lost as time passes. Thus, before the data stored in the capacitor is completely lost, a refresh operation to detect/amplify data and rewrite the data is needed. A DRAM refresh method can be an auto refresh method or a self-refresh method. Since both methods are well known techniques, detailed descriptions thereof will be omitted herein.
When the temperature of the DRAM is high, the loss of electric charges in the cell capacitor becomes faster. Thus, when the temperature of the DRAM is high, a refresh interval needs to be shortened. For this reason, there have been efforts to change the refresh interval according to the temperature. To change the refresh interval according to the temperature, in the conventional technique, a temperature sensor is provided inside the DRAM chip so that the refresh interval is controlled according to the temperature sensed by the temperature sensor.
However, according to the above conventional technique, not only is a complicated temperature sensing circuit required, but also reliability or detection performance of the temperature sensing circuit is not high.